In sheet metal stretch forming processes, a hydraulic press machine is often used to support and move opposing forming tools required to form a flat sheet metal blank into a three-dimensional contoured article or product. The press moves the tools from an open position, in which a finished part is removed and a new blank inserted, to a closed position for stretching the sheet metal blank against the tools to form the product. Large presses for shaping large parts typically open and close along a vertical axis. A vertical press, thus, has a lower platen for supporting one of the tools, often a punch or male form tool, and an upper platen for carrying a complementary, opposing tool with a concave cavity, typically a female tool or die. Often the lower platen is raised by a hydraulically actuated ram to close the press. In hot stretch forming, the tools may be individually heated to maintain a suitable forming temperature for the sheet metal blank and the female tool may simply form a closed chamber against an upper surface or side of the sheet metal blank for introduction of a pressurized working gas to stretch the sheet metal blank against the male tool.
In order to stretch the sheet metal blank between the tools, the edges of the sheet metal blank must be gripped so that the interior part of the sheet metal blank is suitably stretched against a forming tool surface. This gripping function is accomplished by opposing binder surfaces. Depending upon the complexity of the shape of the product to be formed, the binding surfaces may be provided on the margins of the opposing tools, or a separate tool sometimes called a binder ring may provided at the margin of a tool to assist the binder function. Such a binder ring may be movable separately from the tool that it surrounds or with which it cooperates.
FIG. 1 illustrates tooling 10 typically used for hot stretch forming of a sheet blank of an aluminum alloy, e.g., AA 5083 formable at elevated temperatures, e.g., about 450° C. Some hot blow forming processes do not require an upper female forming die, but nonetheless include an upper tool 12 for clamping the sheet metal blank (not shown) about its periphery between the upper tool 12 and a lower tool 14. An upper ram of a press (not shown) may carry the upper tool 12, and a lower platen of the press (not shown) may carry a stationary male form die 16 wherein the lower tool 14 encircles the form die 16 and is either separate therefrom or is integral therewith. The lower tool 14 includes laterally opposed ends 18a, 18b, and laterally opposed sides 20a, 20b, each having corresponding upper surfaces 22a, 22b, 24a, 24b. Likewise, the upper tool 12 includes laterally opposed ends 26a, 26b, and laterally opposed sides 28a, 28b, having corresponding lower surfaces 30a, 30b, 32a, 32b that correspond in kind to the upper surfaces 22a, 22b, 24a, 24b of the lower tool 14.
In operation, the sheet metal blank is placed on top of the contoured surfaces 22a, 22b of the opposed ends 18a, 18b of the lower tool 14. Then, the upper ram of the press drives the upper tool 12 toward the lower tool 14, wherein the sheet metal blank is initially held just between the flat lower surfaces 32a, 32b of the upper tool 12 and the contoured surfaces 22a, 22b of the lower tool 14. As the upper ram of the press continues to drive the upper tool 12 down, the sheet metal blank is first bent into engagement with the flat surfaces 24a, 24b of the lower binder 12 and is eventually bent into complete engagement between the contoured surfaces 22a, 22b of the lower tool 14 and the contoured surfaces 30a, 30b of the upper tool 12. Thereafter, and in accordance with typical Quick-Plastic-Forming (QPF) processes, heating elements (not shown) in the upper tool 12, lower tool 14, and form die 16 heat the sheet metal blank, and pressurized gas is introduced through a port 34 in the side 28a of the upper tool 12. The gas remains pressurized by virtue of a seal created between the upper press platen and an upper surface 36 of the upper tool 12 and by virtue of the seal created by the sheet metal blank which is squeezed between the upper tool 12 and the lower tool 14. As is well-known, the pressurized gas forms the heated sheet metal blank over the form die 16 to create the finished product.
In general, sheet metal that is subjected to a hot gas blow-forming process will undergo thickness reduction, or thinning, depending on factors such as the specific tool surface shape and relative shape and position of the blank. Extreme thinning must be avoided in order for the product to serve its structural purposes. It is also occasionally possible for a complex panel to wrinkle if the blank undergoes compressive stresses sometime during the forming operation. In other words, a finished panel will typically have wrinkles if the surface area of the sheet blank is greater than the final part shape.
In order to avoid the above-mentioned thinning and wrinkling problems, it has been proposed to use more than one forming stage, involving at least one hot blow forming tool. Such an alternative, however, can be cost prohibitive. Also it has been proposed to enlarge an addendum area of the blank, located between the blank holding margin of the blank and the finished component portion of the blank, in order to alleviate the non-uniform stretch condition between the flat clamping surfaces of the lower binder and the contours of the form die. Unfortunately, larger addendum areas increase the size of the blank, thereby leading to increased material costs.
Thus, there is a need to minimize or eliminate wrinkling and thinning conditions in metal forming processes, particularly hot blow forming processes, while avoiding the expense of current solutions to those problems.